This invention is directed to a preferably highly selective high frequency surface acoustic wave (SAW) filter of the dual mode type (DMS-SAW filter or DMS filter). The term xe2x80x9clongitudinal mode resonator filterxe2x80x9d is also used to describe the filter. These DMS filters are used as band pass filters, preferably in cordless or cellular telephones. If a cellular telephone is used as an example, the DMS filters can be located in the HF (high frequency) receiver section or in the transmitter section. For example, in the receiver, DMS filters are positioned between the first low-noise preamplifier (LNA) and a subsequent mixer so that only s filtered signal is converted to the intermediate frequency.
DMS filters are already known, for example, as single-track filters. To provide higher selectivity, filters are also manufactured and used in which two such filter tracks are located on a substrate and combined in a cascaded fashion. DMS filters of this type are known from the European patent document EP-0836278A. FIG. 13 shows a known two-track cascaded filter comprised of two single-track DMS filters connected together.
These embodiments each have resonator/reflector structures at the ends of each respective surface acoustic wave track, and between these at least one interdigital transducer for signal input and at least one for signal output.
FIG. 13 shows a known cascaded filter that includes two acoustic tracks and two single-track filters 1300, 1390 connected together. In this cascaded filter, for example, transducer 1310 is provided as a selective asymmetric/symmetric input to the filter. The connections of the transducer 1360, which is used in this representation as an output, are symmetric outputs (OUT bal and OUT bal). The remaining transducers 1321, 1322, 1371, and 1372 are coupling transducers, as can be seen in the figure, with which the two tracks 1300 and 1390 are electrically coupled together.
As indicated in FIG. 13, these input transducers can be operated symmetrically or asymmetrically, either with a symmetric signal input to both sides (IN bal/IN bal) or an asymmetric input grounded on one side (IN/ground). It should be noted that in a filter of this type, the input and the output can be interchanged or can be used alternatively.
In practice, transducers 1310 and 1360 in FIG. 13 are always designed with mirror-image symmetry about the center plane M, which is perpendicular to the direction of propagation x of the surface acoustic wave. Accordingly, these transducers have an odd number of meshing fingers. In the figure, this involves, for example, five interdigitally arranged fingers on transducers 1310 and 1360.
The foregoing arrangement is preferably used with non-impedance transforming filters. The input impedance is in this case the same as the output impedance, Zin=Zout. Most frequently, the impedance Zin=Zout=50xcexa9 is used.
In the area of application mentioned above as an example, differential mixers with higher impedances are also used (for example, 200xcexa9). However, if the output of the preamplifier (LNA) is to remain 50xcexa9, an impedance transforming filter offers an optimum solution to allow tailoring when there is a minimum number of components.
This impedance transformation canxe2x80x94as is knownxe2x80x94be produced in two ways.
A) From the article xe2x80x9cHigh Performance Balanced Type SAW Filtersxe2x80x9d, by G. Endoh, M. Ueda, O. Kawachi and Y. Fujiwara, IEEE, Ultrasonics Symposium October 1997, it is known that by shrinking the aperture of the track with the output compared to the aperture of the other track, a higher impedance Zout results. A disadvantage of this solution is an increased insertion loss (compared to similar apertures) due to erroneous internal adjustment of the tracks.
B) In order to attain, for example, a 1:4 impedance transformation in a DMS filter made of four individual tracks, two tracks are connected together in parallel at the input and two tracks are connected together in series at the output. The disadvantage to this is its large, extremely complex layout requiring a very large chip surface, which is thus very expensive to manufacture (many bonding wires).
For use as a common impedance transforming asymmetric/symmetric HF filter that is installed prior to a symmetric mixer, it is critical to maintain the required symmetry in the pass band of the band pass filter. The amplitude symmetry of the two output signals a1 and a2, defined as xcex94ampl.=ampl.(a1)xe2x88x92ampl.(a2), must not be more than xc2x11.0 dB (decibels):
xcex94ampl.xe2x89xa6xc2x11.0 dB 
Likewise, the phase symmetry of the two output signals a1 and a2, defined as xcex94xc3x8xe2x88x92180xc2x0 with xcex94xc3x8=xc3x8(a1)xe2x88x92xc3x8(a2), must be below 10xc2x0 in the pass band:
xcex94xc3x8xe2x88x92180xc2x0xe2x89xa6xc2x110xc2x0
Another characteristic of symmetric filters is their high stop-band attenuation. In the ideal case, the two symmetric signals are in phase and of equal magnitude outside the pass band. Any deviation from this ideal case leads to a reduction in the signal suppression. An undesirable residual signal is the result, i.e., the filter has a lower stop-band attenuation (=selection outside of the pass band).
The object of this invention is to produce a filter that has improved symmetry characteristics in the pass band for operation as an asymmetric/symmetric or symmetric/symmetric filter without increasing the required circuitry or the required chip surface.
This object is met by a SAW filter according to claim 1. Advantageous embodiments of the invention can be found in the dependent claims.
This invention is based on a DMS filter with at least one track on a piezoelectric substrate with an odd number of first interdigital transducers and an even number of second interdigital transducers that are connected to the input and the output of the filter and are arranged between reflector structures. In at least one acoustic track, the middle transducer is split symmetrically into partial transducers with respect to the perpendicular of the direction of propagation of the surface acoustic wave, which leads to an even number of electrode fingers for this middle transducer. In addition, the filter is connected in a symmetrical manner to the connection pads or connection pins of a housing so as to provide an axis-symmetrical connection layout.
It is the splitting of the middle transducer, i.e., the transducer among the odd number of first interdigital transducers that is located in the middle of the track, that allows an axis-symmetrical connection to the connections of the housing, which in turn are also arranged axis-symmetrically.
By increasing the geometric symmetry, the symmetry of the transmission behavior of the filter is also improved in the pass band for the following three cases: an impedance transforming asymmetric/symmetric filter, an impedance transforming symmetric/symmetric filter, and a non-impedance transforming symmetric/symmetric filter.
It is also possible to split the middle transducer symmetrically into partial transducers in at least two acoustic tracks. In the process, the sum of the number of fingers of the split partial transducers located in one acoustic track is even, whereas the number of fingers of each partial transducer can be even or odd. Preferably, in cascaded multi-track filters according to the invention, the two outer tracks that are connected to the input or output have split middle transducers. In a filter operated in symmetric/symmetric mode, this leads to a further improvement in the transmission behavior.
Also advantageous to symmetric transmission behavior is if the housing is connected by means of bump connections. This prevents asymmetries caused by parasitic capacitances and inductivities that can result from what are often different lengths of bonding wires. This is practically unavoidable in wire bonding. The result is that the filter is preferred to be installed in the housing using flip chip technology. Bump connections can be produced more regularly and produce fundamentally fewer parasitic capacitances and inductivities.
It is also advantageous if non-split middle transducers or other interdigital transducers have an even number of electrode fingers.
In the asymmetric/symmetric operation of the filter, the asymmetric gate (input or output) has a single signal-carrying connection and an associated ground connection. The symmetric gate has two signal-carrying connections. An ideal symmetric housing with five connections (three of these being signal-carrying connections) is advantageous for this purpose, with the connection pad for the signal-carrying connection lying on the asymmetrical gate on the axis of symmetry and the connection pad for the connection pair lying at the symmetric gate and the pad for ground being symmetric with respect to the axis of symmetry. Two ground connections have the advantage in that the ground connection from the chip to the housing and to the outside can also be designed symmetrically.
In symmetric/symmetric operation of the filter with pairs of connections, an ideal symmetric housing with six connections is advantageous, with two connection pads each being arranged symmetrically with respect to the axis of symmetry for ground, the input, and the output.
It is preferable to arrange the connection pads for ground between the connection pads for input and output. This allows a better capacitive separation, thus reducing any undesirable coupling of input and output (direct cross-talk).
In one embodiment of the invention, the connection pads for ground are combined into a common pad that is symmetric with respect to the axis of symmetry.
To tailor it to a particular circuit environment, the filter can be impedance-adjusted and, in particular, impedance transforming. These measures can be undertaken in a filter according to the invention in a known manner. It is preferable for the impedance transformation factor to be 1:4. In using different apertures in the two tracks (see embodiment A), it is also possible to use modified impedance transformation factors, but the insertion loss increases in this case as already mentioned.
The invention is explained below in more detail with the help of exemplary embodiments and 14 figures associated therewith. These show the following in partially schematic and non-scaled representation: